Such a generalising statement could be misleading.
I heard from D500 users how badly Wifi was implemented in their cams.
On the other hand I couldn't be happier with the D750 style of implementation.
I found the D750 wifi useless to my purposes since it disconnects if the camera is turned off and is turned back on, the connection has to be re-established from the phone (settings / wifi etc.). IMO the connection should be established once and then the devices should stay paired until the user explicitly request disconnection (data doesn't necessarily have to be transferred while the camera is off but it should start transmitting when the camera is turned back on). I don't want to select images to transmit; I need to have the latest images transmitted in the background so I can always show and evaluate them as soon as possible. Furthermore the mobile device should preferably stay connected to the internet at the same time and this connection should not be affected by the connection to camera. If I connect the mobile device to D750 wifi all internet access from the phone or tablet seems to be lost.
I do not need transfer of high resolution or raw files to the phone or tablet; what I want to do is shoot in the studio and display the images at the same time on a larger screen (iPad, 2K images suffice or slightly bigger would be fine, but not full resolution if it slows the transmission) so that I can evaluate the lighting when setting up and doing test shots and also so that the subject or client can see the images with a good display (the iPad retina display may not be calibrated but it's close enough to do this job better than using the LCD on the back of the camera). I don't want a wired connection since my experience is that tripping from cables is more likely than not and it could lead to substantial damage. I just need the connection to be totally transparent when I'm shooting and it must not be disconnected if I turn the camera off for a while.
I have read that Android devices are capable of automatically reconnecting to the phone if the camera power is recycled. However, this doesn't seem possible on iOS devices which is what I use.
Nikon is trying to solve these problems with the Snapbridge implementation where the bluetooth is supposed to stay active and trasmit (if slowly) small images while the camera is off. The faster wifi is reported to be disconnected but since I do not need high res images on the mobile device this would be fine. What's crucial to me is that I don't have to explicitly go to the device menus to reactivate the connection all the time. This totally kills the user experience for me. The images should simply appear on the mobile device while I use the camera normally.
If I want high resolution images then I would use a laptop and a cabled connection and inform the people at the shoot how important it is not to touch the cable when moving around at the set at any time. However, I don't typically need to see fine details at this point. I have a Manfrotto digital director which is great for using an iPad as a live view screen (e.g. for video) but I find usability problems with that as well. The camera controls do not work if the digital director is active. With Camranger I find the screen lag to be too long. None of the solutions for the tethered display I've worked with have been good, to be honest. For camera remote control, several products seem to work well though, but this is not my main interest in tethering. I have not tried WT-6A for the D5. I read that on the D4 the WT-5 connection is not only lost if the camera is turned off, but also if the camera goes into standby mode! (I'm rolling my eyes.)